1. Field of the Invention
This invention relates to a switching power circuit.
2. Description of Related Art
FIG. 5 is a block diagram showing a switching power circuit having a conventional MAGAMP magnetic amplifier system regulator. In FIG. 5, symbol T1 denotes a switching power supply transformer, which is comprised of a primary winding N11, secondary windings N21, N22, and a primary auxiliary winding N12 supplying power to a primary control circuit. A commercial power supply Vin (AC) is rectified by a diode bridge DB1, and is smoothed by a smoothing condenser C1 to thereby acquire a direct current voltage Vin (DC). A field effect transistor (FET) Q1 performs high-frequency switching of the direct current voltage Vin (DC) charged in the condenser C1, and the direct current voltage Vin is applied to the primary winding N11 of the transformer T1. A pulse voltage synchronous with an output of the primary winding N11 is generated at the secondary winding N21 of the transformer T1. A rectifier circuit formed of diodes D11, D12, a choke coil L11 and a condenser C11 rectifies and smoothes the pulse voltage to thereby acquire a direct current voltage Vo1.
Resistances R11, R12, R13, a Zener diode Q3, a photo coupler Q2 and the primary control circuit detect and feedback-control the direct current voltage Vo1 by controlling the on/off time ratio for switching the field effect transistor Q1 in such a manner that the direct current voltage Vo1 can be a desired value. At the second winding N22, a saturable reactor L22, a transistor Q22, a Zener diode Q21, resistances R21, R22, R23, R24, R25 and a diode D23 control a direct current voltage Vo2 to a desired voltage by a MAGAMP (Magnetic Amplifier) system. A forward-type rectifier circuit is comprised of a rectifier diode D21, a flywheel diode D22, a choke coil L21 and a smoothing condenser C21.
The MAGAMP system is based upon a magnetic saturation operation of the saturable reactor. The saturable reactor is a device which has a sufficient initial inductance and is magnetically saturated to have an inductance L≈0 when an integrated value of a certain voltage * a time (which is generally called the xe2x80x9cET integrated valuexe2x80x9d) is applied to the saturable reactor. In FIG. 5, symbol L22 denotes the saturable reactor. FIG. 6 is a waveform chart showing waveforms at points A and B in the switching power circuit in FIG. 5. As shown in FIG. 6, when a voltage V1 is generated at the point A at a time T0, the impedance of the saturable reactor L22 is high until a predetermined time Ti, so that no voltage is generated at the point B. When the ET integrated value (V1 * (T1xe2x88x92T0) reaches a saturation ET integrated value of the saturable reactor L22 at the time T1, the impedance of the saturable reactor L22 is decreased to such a low value that the voltage at the point A passes through the point B. A reset current Ir is carried through the saturable reactor L22 through the transistor Q22 between times T2 and T3, so that the saturable reactor L22 is reset or released from its saturated state. Thereafter, the same process (the high impedance, the decrease in impedance, and the reset) is repeated from the time T3.
The saturation ET integrated value of the saturable reactor L22 can be controlled by controlling the reset current Ir. Specifically, the resistances R21, R22 detect the output voltage Vo2, and the reset current Ir corresponding to a difference of the detected output voltage Vo2 from a reference voltage is carried through the saturable reactor L22, thus stabilizing the output voltage Vo2 at a desired voltage. This is called the MAGAMP system regulator.
A synchronous rectifying system may be used instead of the MAGAMP system. A description will now be given of a synchronous rectifier circuit with reference to FIG. 7. FIG. 7 is a block diagram showing a switching power circuit provided with a synchronous rectifying system regulator. Field effect transistors (FET) Q20, Q21 as semiconductor switching devices are connected in parallel with secondary rectifier diodes D21, D22, respectively. The field effect transistor Q20 is turned on only while the diode D21 is conducted. The field effect transistor Q21 is turned on only while the diode D22 is conducted. Consequently, a current is carried through the field effect transistors Q20, Q21 with low ON resistance, and the current is rectified by a drop in voltage by a forward voltage Vf (≈0.5V) of the diodes D21, D22 or less.
Since a certain limited time is required for turning on/off the field effect transistors Q20, Q21, there is a time-lag when drive signals for the field effect transistors Q20, Q21 are acquired from a drive signal for the primary field effect transistor Q1 in synchronism therewith. To address this problem, a PWM control circuit detects the output voltage Vo1 regarded as a reference oscillation signal S0, and a signal S1 with a time lag from the reference oscillation signal S0 drives the field effect transistor Q1. A signal S2 with a smaller time lag than the signal S1 drives the synchronous rectifier field effect transistors Q20, Q21. This is called the synchronous rectifying system.
Thus, the synchronous rectifying system requires a complicated synchronous signal circuit. Further,. an additional circuit is needed for acquiring the drive signal for the primary switching device (field effect transistor Q1). Therefore, the synchronous rectifying system is not suitable for a method wherein a control IC is arranged at an upstream side (see FIG. 5), which is now the mainstream.
In recent years, the operating voltage of digital ICs has been lowered, and the direct current voltage Vo2 of the above-mentioned switching power circuit is set to 3.3V in many cases. In such cases, a power loss (=Vf * Io) due to the forward voltage Vf (≈0.5V) of the diodes D21, D22 is relatively larger than in the case where the voltage Vo2 is 5V. This deteriorates the power conversion efficiency.
It is therefore an object of the present invention to solve the above-mentioned technical problems.
It is another object of the present invention to provide a switching power circuit, which has improved power conversion efficiency with a simple structure and low costs.
To accomplish the above objects, according to a first aspect of the present invention, there is provided a switching power circuit comprising a transformer having a plurality of windings, one end of a primary winding of the transformer being connected to a voltage source, a first switching device, another end of the primary winding being connected to a return side of the voltage source through the first switching device, a magnetic amplifier connected to the secondary winding of the transformer, a forward-type rectifier circuit connected to the magnetic amplifier and including at least a flywheel diode, a second switching device connected in parallel with the flywheel diode, and a control circuit for turning on/off the second switching device according to an output of the secondary winding of the transformer or according to a signal acquired by inverting the output of the secondary winding.
To accomplish the above objects, according to a second aspect of the present invention, there is provided a switching power circuit comprising a transformer having a plurality of windings, one end of a primary winding of the transformer being connected to a voltage source, a first switching device, another end of the primary winding being connected to a return side of the voltage source through the first switching device, a semiconductor switching device connected to the secondary winding of the transformer, a forward-type rectifier circuit connected to the semiconductor switching device and including at least a flywheel diode, a second switching device connected in parallel with the flywheel diode, and a control circuit for turning on/off the second switching device according to an output of the secondary winding of the transformer or according to a signal acquired by inverting the output of the secondary winding.
Preferably, the switching power circuit according to the second aspect further comprises a synchronous chopper control circuit for turning on/off the semiconductor switching device according to an output of the forward-type rectifier circuit.
To accomplish the above objects, according to a third aspect of the present invention, there is provided a switching power circuit comprising a transformer having a plurality of windings, one end of a primary winding of the transformer being connected to a voltage source, a first switching device, another end of the primary winding being connected to a return side of the voltage source through the first switching device, a conducting/cutting-off circuit connected to the secondary winding of the transformer, for conducting and cutting-off an input signal, a forward-type rectifier circuit connected to the conducting/cutting-off circuit and including at least a flywheel diode, a second switching device connected in parallel with the flywheel diode, and a control circuit for outputting a control signal for controlling conducting or cutting-off timing of the conducting/cutting-off circuit according to an output of the forward-type rectifier circuit, and wherein the second switching device is controlled in such a manner as to be off during a predetermined period included in a cutoff period of the conducting/cutting-off circuit.
Preferably, the conducting/cutting-off circuit comprises a saturable reactor, and the control circuit comprises a reset current control circuit for controlling a reset current for resetting the saturable reactor.
Alternatively, the conducting/cutting-off circuit comprises a semiconductor switching device, and the control circuit comprises a synchronous chopper control circuit for turning on/off the semiconductor switching device.
In a preferred form of each aspect, the forward-type rectifier circuit further comprises a rectifier diode, a choke coil, and a smoothing condenser.
In a preferred form of each aspect, the transformer further comprises a second secondary winding, the switching power circuit further comprising a second forward-type rectifier circuit connected to the second secondary winding.
In a preferred form of each aspect, the switching power circuit further comprises a second control circuit for turning on/off the first switching device according to an output of the second forward-type rectifier circuit.
According to the switching power circuit of the present invention, to obtain a power supply output using the MAGAMP or the semiconductor switching device connected to the secondary side of the transformer of the switching power source, a primary switching signal is acquired from the output of the secondary winding. The second switching device (rectifier field effect transistor at the flywheel side) is driven according to a drive signal acquired from the primary switching signal or by inverting the primary switching signal. This realizes a switching power circuit having a regulator of a relatively-low voltage which has improved power conversion efficiency with a simple structure and at low costs.